The invention relates to an apparatus for ultrasonic wire bonding, whereby bonding loci can be interconnected by wire straps, by means of ultrasonic welding. For example, the apparatus may be used to connect terminals on component mounts to bonding islands on semiconductor chips.
In known apparatuses for ultrasonic wire bonding, as a rule the wire to be welded is fed through an inclined bore in the longitudinal direction of the ultrasound generator, which bore is disposed behind the welding surface of a sonotrode. This customary arrangement requires that prior to the fixing of each wire strap the connecting orientations at the two connecting loci of the strap must be aligned with the axial direction of the ultrasound generator.
For this purpose, either the component mount or a bonding head which includes the ultrasound generator must be aligned in the required direction. When automatic apparatus is used for certain areas of application, e.g. for operations on so-called hybrid components, it is advantageous to proceed by adjusting the bonding head.
Known apparatuses of this type have a bonding head which is rotatable around a vertical axis which axis approximately passes through the welding surface at the foot of the sonotrode, and the bonding head as a rule is also movable vertically. In addition, all known embodiments have an optical device disposed near the axis of rotation, whereby the component being operated on can be depicted on a monitor by means of a television camera. In these apparatuses the wire is generally fed through a bore in the bonding snout which bore is inclined to the horizontal at an angle of approximately 30.degree. to 60.degree.. Ordinarily, pincers are provided for the automatic advancing and cutting of the wire, whereby the wire is clamped in two pincer jaws behind the sonotrode. The wire is advanced under the welding surface of the sonotrode prior to the execution of the first welded connection by advancing the pincer jaws in the longitudinal direction of the wire being fed, in a manner coordinated with the clamping movements of the said pincer jaws. Then after the second welded connection of the wire strap has been executed, the wire is severed at the rear edge of the welding surface of the sonotrode.
In known apparatuses having a rotatable and vertically movable bonding head, the supply reel for the wire is disposed on the bonding head. This arrangement necessitates the use of a relatively small and light wire supply reel, in order to afford the free space required particularly for the rotation of the bonding head, and to minimize the moment of inertia around the rotational axis.
There are apparatuses known in the art (described, e.g., in U.S. Pat. No. 4,239,144) wherein an ordinary light reel having a diameter of approximately 12.5 mm and length of 19 mm is rotatably inserted on a shaft on the bonding head. When the wire strap is being formed, the wire is unreeled from the reel. A disadvantage of this arrangement is the need to accelerate and rotate the reel body. This causes fluctuations in the wire tension. As a result it is difficult to reliably provide a test voltage on the bonding wire, which voltage is needed for electrical monitoring of bond quality. A further disadvantage is that the reels used are unsuitable for wires of diameter greater than 0.05 mm.
Also known in the art is an apparatus wherein the wire is withdrawn from the end face of a reel body which is held fixed on the bonding head. The reel body is smaller than the usual reel body used for this type of wire feeding. Disadvantages of this arrangement are that the amount of wire which can be held on one reel is less and the reels used are suitable only for thin wires. Further, the space requirements for the reels and their mountings are relatively large.